Abstract: The paper points out that the water drive curves could be applied to the oil fields in which the main part of the 1g(Kro/Krw)~Sw curve is straight line. Certainly proved the main part of the lg (Kro/Krw)~Sw curve is real linear for Changhuan internal and skirt reservolrs of Daqing Oil Field and water drive relation curve can be used to analyze their per formance, it demonstrates that the real linear part of the water drive curve may appear be fore/after 50% water-cut in theory and for practlcal examples of Daqing Oil Field. The appearing point of straight line depends on characteristics of forrnation and fluid. The paper also gives a method to determine the appearing point for real straight line by combination of lg (Kro/Krw)~Sw relation and two different types of water drive curves.